1. Field of the Invention
The present invention relates to a communication system using communication channels, one of which transmits a signal in a state where its optical power is relatively small, and the other of which transmits a signal in a normal state where the optical power is relatively large. In particular, the present invention relates to a method of establishing synchronization between communicating devices in the communication system.
2. Description of the Related Art
In the field of quantum cryptography, it is known, based on Heisenberg's uncertainty principle, that eavesdropping between a sender and a receiver can be detected with high probability. In other words, this fact allows the sender and receiver to share a secret bit string (cryptographic key) without being eavesdropped. As an example of a procedure to share the secret information, BB84 (Bennett Brassard 84) protocol using four quantum states is known. A high level of security can be achieved by using a bit string created through this procedure as a key of Vernam cipher, which has been proved to be perfectly secure. The key-sharing procedure according to the BB84 protocol will be described briefly.
FIG. 1 is a schematic diagram for explaining a key-sharing procedure, steps 1 to 8, according to the BB84 protocol. The key-sharing procedure is as follows:                Step 1) a sender creates phase-modulating data based on random data bits, which are source data of a cryptographic key, and on random data, which is base information A (“+” base or “×” base) to be used at modulation. The sender stores the phase-modulating data;        Step 2) the sender modulates the phase of an optical pulse based on the phase-modulating data and sends the phase-modulated optical pulse to a receiver through a quantum channel;        Step 3) the receiver receives the optical pulse from the sender via an interferometer while also modulating the phase of the received optical pulse based on random base data (“+” base or “×” base);        Step 4) the receiver stores data bits that the receiver could optically receive through the interferometer, that is a detected output, and base information B used at the detected output. The receiver sends the base information B to the sender through a classical channel;        Step 5) the sender compares the base information B received from the receiver with the base information A stored. From the random data bits that are the source data, the sender discards bits whose corresponding bases in the base information A do not match the corresponding bases in the base information B;        Step 6) the sender sends the bit numbers of the remaining bits, which have not been discarded, to the receiver through the classical channel;        Step 7) the receiver discards the detected data bits with bit numbers other than the bit numbers received from the sender; and        Step 8) the data bits that remain in the end are made cryptographic key data to be shared between the sender and the receiver.        
There have been proposed some quantum key distribution systems employing such a key-sharing scheme. In particular, “Plug & Play” schemes proposed by groups at the University of Geneva, Switzerland, are supposed to be promising schemes to bring into practical use a quantum key distribution system, which is sensitive to polarization, because the “Plug & Play” schemes can compensate a polarization drift occurring over an optical fiber transmission line. (See the followings:                G. Ribordy, J. D. Gautier, N. Gisin, O. Guinnard, and H. Zbinden “Automated ‘plug & play’ quantum key distribution” ELECTRONICS LETTERS, Vol. 34, No. 22 (Oct. 29, 1998), pp. 2116 to 2117;        A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden, and N. Gisin “‘Plug & Play’ systems for quantum cryptography” Applied Physics Letters, Vol. 70, No. 7 (Feb. 17, 1997), pp. 793 to 795; and        H. Zbinden, J. D. Gautier, N. Gisin, S. Huttner, A. Muller, and W. Tittel “Interferometry with Faraday mirrors for quantum cryptography” ELECTRONICS LETTERS, Vol. 33, No. 7 (Mar. 27, 1997), pp. 586 to 588.)A general configuration of a “Plug & Play” system is shown in FIG. 2.        
As shown in FIG. 2, at a receiver (traditionally referred to as “Bob”) which is to receive a quantum cryptographic key in the “Plug & Play” system, an optical pulse P is first generated by a laser LD and then split into two pulses. One of the pulses, an optical pulse P1, is allowed to go along a short path, and the other, an optical pulse P2, is allowed to go along a long path, whereby the two pulses are sent to a sender (traditionally referred to as “Alice”) sequentially with a small time delay between them. Upon receiving the optical pulses P1 and P2 sequentially, Alice allows the optical pulse P1 to be reflected by Faraday mirrors to make its polarization state rotate by 90 degrees and sends the optical pulse P1 back to Bob. Moreover, Alice similarly allows the optical pulse P2 to be reflected by the Faraday mirrors while modulating the phase of the optical pulse P2. Then, Alice sends a phase-modulated optical pulse P2*A back to Bob.
Bob allows the optical pulse P1 received from Alice to pass along the long path, which is a different path from the path used when the optical pulse P1 was sent out. At the same time, Bob modulates the phase of the optical pulse P1 to obtain a phase-modulated optical pulse P1*B. Meanwhile, the optical pulse P2*A, which has been phase-modulated on the Alice's side, is allowed through the short path, which is a different path from the path used when it (i.e., the optical pulse P2) was sent out. Thereafter, the optical pulse P2*A is made to interfere with the optical pulse P1*B, which has been phase-modulated on the Bob's side. The result thereof is detected by an optical detector APD1 or APD2. As a whole, the optical pulses P1 and P2, obtained by splitting an optical pulse into two on the Bob's side, follow the same optical path and then interfere with each other. Accordingly, since variations in delay due to the optical fiber transmission line cancel out, the result of the interference observed by the optical detector depends on a difference between the phase modulation on the Alice's side and the phase modulation on the Bob's side.
The “Plug & Play” system having such a configuration requires synchronization as described below:                1) at the sender (Alice), to modulate the optical pulse P2 received from the receiver (Bob), the optical pulse P2 should be made to follow the variations in delay due to the optical fiber transmission line;        2) at the receiver, to modulate the optical pulse P1 reflected from the sender, the optical pulse P1 should be made to follow the variations in delay due to the optical fiber transmission line; and        3) at the receiver, when any of the optical pulses is received, a bias to apply to the optical detector should be applied in accordance with the received optical pulse (ultra-high-sensitivity reception in Geiger mode).        
Moreover, in quantum key distribution systems, it is also required to establish the synchronization of bit positions for basic information exchanges and for a key creation sequence.
In the quantum key distribution systems, however, unlike a conventional optical communications system, the optical power is very small, at a single photon level at most. Therefore, it is impossible to perform, using a quantum channel, clock extraction as in a conventional case of using a classical channel. In the present disclosure, a quantum channel is defined as a communication channel in a state where the optical power of transmission from a sender to a receiver is very weak, with at most one photon per bit, whereas a classical channel is defined as a communication channel that is in a normal optical power range.
Specifically, when communications are performed using a quantum channel with light at a very low optical power level, the light hardly reaches an optical detector APD. Therefore, for example, even if a sender sends data with a mark ratio of 1/2, the mark ratio becomes far smaller than 1/2 at a receiver. Accordingly, data losses occur, and an accurate-period clock signal cannot be extracted. A classical channel is therefore normally used to provide synchronization for such a quantum channel.
For example, Japanese Unexamined Application Publication No. H08-505019 discloses a method to provide bit synchronization and other system calibration using a classical channel. According to this method, both the quantum channel and classical channel are installed in the same transmission line, and the classical channel is used to provide clock synchronization for the quantum channel where the optical power is very weak.
Further, Japanese Unexamined Application Publication No. S63-107323 discloses an optical transmission system in which variations in the transmission characteristics of an optical fiber are detected on the receiver side, thereby making it possible to compensate the characteristic variations. Specifically, a reference signal, which has a different wavelength from that of a data signal, is sent from a receiver to a sender and returned by the sender to the receiver. From this returned reference signal, variations in the transmission characteristics of the optical fiber are detected.
However, in practice, there is wavelength dispersion in an optical transmission line. As described in Japanese Unexamined Application Publication No. H08-505019, even in the same transmission line, the propagation times over the quantum channel and the classical channel are different from each other because of the different wavelengths used in the quantum channel and the classical channel. Accordingly, the phase relationship between signals over the quantum channel and over the classical channel becomes out of phase, and it is therefore impossible to establish the clock synchronization over the quantum channel and the bit synchronization for key creation. If the quantum channel and the classical channel are individually formed of separate transmission lines, a setup is needed for each of the transmission lines because the propagation time depends on the characteristics (propagation length, dispersion, etc.) of the individual transmission line. In any case, the phase relationship between signals over the quantum channel and over the classical channel is out of phase, and it is therefore impossible to establish the clock synchronization over the quantum channel and the bit synchronization for key creation.
In addition, also in the optical transmission system disclosed in Japanese Unexamined Application Publication No. S63-107323, the different-wavelength reference signal is transmitted to compensate fluctuations of an optical signal. Therefore, from a similar reason, it is impossible to extract a clock signal from the quantum channel and to transmit with accuracy a reference signal for providing key creation synchronization, that is, for providing the synchronization of bit positions.